1. Field of the Invention
The present invention relates to an optical head for recording and/or reading information on and/or from a magneto-optical record medium.
2. Related Art Statement
In an optical head for recording and/or reading information on and/or from a magneto-optical record medium, a linearly polarized laser beam emitted by a semiconductor laser is made incident upon the magneto-optical record medium as a fine spot by means of an objective lens, a return beam reflected by the magneto-optical record medium is separated into two beams polarized in orthogonal directions, and these two return beams are received by photodetectors. By suitably processing output signals from these photodetectors, an information signal, a focusing error signal and a tracking error signal are derived. The later error signals are required for correcting a relative positional deviation between the objective lens and the magneto-optical record medium.
In Japanese Patent Application Laid-open Publication Kokai Sho 63-161541, there is proposed a known optical head, in which the return beam reflected by the magneto-optical record medium is divided into the two orthogonally polarized beams by using a composite prism element and the information signal, focusing error signal and tracking error signal are derived by the output signals produced from the photodetectors receiving the two beams.
FIG. 1 shows a construction of the known optical head disclosed in the above mentioned Kokai Sho 63-161541. In this optical head, a linearly polarized laser beam emitted by a semiconductor laser 251 is made incident upon a composite prism element 253 by means of a collimator lens 252. The laser beam reflected by the composite prism element 253 is projected onto a magneto-optical record disk 255 by means of an objective lens 254. A return laser beam reflected by the magneto-optical disk 255 is made incident upon the composite prism element 253 by means of the objective lens 254 and is divided into a first beam L.sub.1 and a second beam L.sub.2 whose polarizing directions are orthogonal to each other. The first beam L.sub.1 is called a P-polarized beam and the second beam L.sub.2 is called an S-polarized beam. These beams L.sub.1 and L.sub.2 are made incident upon a photodetecting unit 257 by means of a common converging lens 256 and are received by respective photodetectors. Output signals from the photodetecting unit 257 are supplied to a signal processing unit 258 and are processed thereby to derive information signal S.sub.i, focusing error signal S.sub.f and tracking error signal S.sub.t.
The composite prism element 253 includes a glass prism 259, a quartz prism 260 and a dielectric multilayer film 261 provided on a surface of the glass prism by evaporation, the quartz prism and dielectric multilayer film being cemented to each other by means of an adhesive layer 262 provided on a surface of the quartz prism.
The optical head shown in FIG. 1 will be further explained in detail. In the above mentioned publication, the function of the composite prism element has been explained as follows: The laser beam emitted by the semiconductor laser 251 is converted into a parallel beam by the collimator lens 252 and is made incident upon the glass prism 260 of the composite prism element 253 as S-polarized beam. Then the S-polarized beam is reflected by the dielectric multilayer film 261 and exits from the glass prism 259. The return beam reflected by the magneto-optical disk 255 is made incident upon the glass prism 259 of the composite prism element 253 and is transmitted through the dielectric multilayer film 261, so that the return beam is made incident upon the quartz prism 260 as P-polarized beam.
In the above Publication, there is further explained that from the P-polarized return beam being incident upon the dielectric multilayer film 261, the first beam of the P-polarized component is obtained to have an optical axis Ie which is inclined by .theta.e with respect to a normal line to the boundary plane, i.e. the dielectric multilayer film 261 viewed in a plane y (which contains an optical axis of the incident beam and an optical axis of the return beam, so that the plane y is parallel with the P-polarized plane) and similarly the second beam of the S-polarized beam is obtained to have an optical axis Io which is inclined by .theta.o (.theta.o&gt;.theta.e) with respect to the normal line to the boundary plane viewed in the plane y.
The first P-polarized beam L.sub.1 and second S-polarized beam L.sub.2 are made incident upon respective light receiving element groups of the photodetecting unit 257 by means of the common converging lens 256. By comparing the output signals derived from the first and second beams L.sub.1 and L.sub.2, a rotation of the polarizing plane of the laser beam caused by a vertical magnetizing film of the magneto-optical disk 255 can be detected. In this manner, the information signal S.sub.i can be reproduced.
A change of the return beam due to a tracking servo control pit arrangement on the magneto-optical record disk 255 is detected to derive the tracking error signal S.sub.t, and a shape of the beam spot on the photodetecting unit 257 is detected to derive the focusing error signal S.sub.f.
As described in the above mentioned Publication, it is assumed that output signals from four light receiving regions of the photodetector receiving the first beam L.sub.1 are denoted as Ra, Rb, Rc and Rd and output signals from four light receiving regions of the second photodetector receiving the second beam L.sub.2 are represented by Re, Rf, Rg and Rh. Then, the focusing error signal S.sub.f is expressed by [(Ra+Rd)-(Rb+Rc)]+[(Rc+Rh)-(Rf+Rg)].
As explained above, in the known optical head shown in FIG. 1, the laser beam emitted by the semiconductor laser 251 is converted into the parallel beam by means of the collimator lens 252 and the parallel beam is made incident upon the glass prism 259 as the S-polarized beam. Then, the optical axis of the incident beam is changed by the dielectric multilayer film 261 and exits from the glass prism 259. The return beam reflected by the magneto-optical disk 255 is made incident upon the glass prims 259 and the P-polarized component transmitted through the dielectric multilayer film 261 is made incident upon the quartz prism 260.
In the above Publication, there is not explained at all why the S-polarized beam is changed into the P-polarized beam in the return beam. When the linearly polarized beam is made incident upon the vertically magnetizing film of the magneto-optical record disk 255, the polarizing direction of the beam is rotated or modulated in accordance with the magnetizing pattern related to the information. However, this rotation angle is very small such as about .+-.1.degree., so that the S-polarized beam could not be changed into the P-polarized beam. It should be noted that the polarizing direction of the S-polarized beam differs from that of the P-polarized beam by 90.degree.. Such a large rotation angle of the polarizing direction could not be obtained by the Kerr rotation.
Now a correct operation of the known optical head illustrated in FIG. 1 will be analyzed. The polarizing direction of the S-polarized return beam reflected by the magneto-optical record disk 255 is rotated by .+-..theta..sub.k (.theta..sub.k .congruent.1.degree.). Therefore, the return beam contains both the S-polarized component and P-polarized component, an amount of the S-polarized component is much larger than that of the P-polarized component. In this case, the information can be reproduced by detecting in which direction +.theta..sub.k or -.theta..sub.k the polarization plane is rotated.
In the above mentioned Publication, there is explained that in the known optical head shown in FIG. 1, from the P-polarized return beam, the first P-polarized beam is obtained to have the optical axis Ie which is inclined by .theta.e with respect to the normal line to the boundary plane viewed in the plane y (parallel with the P-polarized plane) and the second S-polarized beam is obtained to have the optical axis which is inclined by .theta.o (.theta.o&gt;.theta.e) with respect to the normal line to the boundary plane viewed in the plane y. It could not be understood why the S-polarized beam can be obtained from the incident P-polarized beam. If this explanation is correct, the S-polarized component and P-polarized component of the return beam is separated from each other, and thus importance information about the direction of the rotation of the polarizing plane is completely lost. That is, it could not be detected in which direction +.theta..sub.k or -.theta..sub.k the polarizing plane has been rotated. Therefore, it is impossible to reproduce the information by comparing the output signals from the first and second photodetectors.
Moreover, in general, when the S-polarized beam and P-polarized beam are made incident upon the quartz prism 260, each of these beams is separated into the ordinary light and extraordinary light. The quartz prism 260 has different refractive indices for the ordinary light and extraordinary light. Therefore, it is practically impossible to combine the P-polarized component into the first beam L.sub.1 and to combine the S-polarized component into the second beam L.sub.2. Furthermore, both of the optical axes Ie and Io of the first and second beams could not be on the plane y.
In the above Publication, there is described that the focusing error signal Sf is derived from [(Ra+Rd)-(Rb+Rc)]+[(Rc+Rh)-(Rf+Rg)]. It is apparent that the fifth term Rc is not correct and should be amended into Re. Nevertheless it is questionable how to obtain the focusing error signal from the above equation. This equation corresponds to a well known equation for deriving the focusing error signal by the astigmatism method. However, in the Publication there is not explained at all how to introduce the astigmatism.
In FIG. 4 of the above Publication, there is shown the photodetecting unit having the first and second photodetectors each comprising the four light receiving regions. In this FIG. 4, a center of the four light receiving regions of each of the two photodetectors are shown to be positioned on the plane y. However, as stated above, both of the optical axes Ie and Io of the first and second beams L.sub.1 and L.sub.2 could not be positioned on the same plane y, both the optical axes Ie and Io could not be positioned on the plane y.
In the known optical head shown in FIG. 1, the tracking error signal St is derived by utilizing the tracking servo control point array provided on the magneto-optical record disk 255. However, this method is unusual and thus could not be applied to general magneto-optical record disk.
As explained above in detail, the known optical head described in the Japanese Patent Application Laid-open Publication Kokai Sho 63-161541 could not be used as a practical optical head or could be used only with difficulty.
In Japanese Patent Application Laid-open Publication Kokai Hei 5-334760, there is disclosed another known optical head having a similar construction as that of the above mentioned known optical head illustrated in FIG. 1. FIG. 2 is a schematic view depicting a simplified construction of the known optical head described in the Kokai Hei 5-334760. A linearly polarized laser beam emitted by a semiconductor laser 270 is made incident upon a first prism 271 and is reflected by a polarizing film 273. An optical axis of the incident beam is then changed and the laser beam is made incident upon a magneto-optical record disk 275 by means of an objective lens 274 as a fine spot.
The return laser beam reflected by the magneto-optical record disk 275 is made incident upon the polarizing film 273 by means of the objective lens 274 and first prism 271. The return beam is separated from the incident beam and is made incident upon a second prism 272 by the polarizing film 273. Then, the return beam is divided into an ordinary light .alpha. and an extraordinary light .beta.. The ordinary light and extraordinary light emanating from second prism 272 are received by first and second photodetectors 277 and 278, respectively arranged on a substrate 276 which is in parallel with an exit surface of the second prism 272.
The first prism 272 is made of a glass material BK-7 having a refractive index of 1.51 and the second prism 272 is made of a quartz whose refractive index for the ordinary light is 1.54 and whose refractive index for the extraordinary light is 1.55. An optic axis of the quartz of the second prism 272 is set to be inclined by 45.degree. with respect to a plane x-y. The polarizing film 273 has a transmissivity of 100% for the P-polarized component and a transmissivity of 20% for the S-polarized component. Therefore, 80% of the S-polarized component is reflected by the polarizing film. For the time being, it is assumed that an apex angle .gamma. of the second prism 272 is set to 90.degree.. In this known optical head, the information signal is reproduced by the differential method, the tracking error signal is derived by the push-pull method, and the focusing error signal is detected by the astigmatism method.
In FIG. 2, an angle between the ordinary light .alpha. and the extraordinary light .beta. becomes 0.56.degree.. Then, spot diagrams of these ordinary light and extraordinary light on the photodetectors 277 and 278 become as shown in FIGS. 3A and 3B. As can be understood from FIGS. 3A and 3B, size of the spots of the ordinary light .alpha. and extraordinary light .beta. on the photodetectors becomes very small such as about 10 .mu.m. In FIGS. 3A and 3B, a length of respective sides of the assembly of the first and second prisms 271 and 272 is 3 mm, a distance between the exit surface of the second prism 272 and the photodetectors 277 and 278 is about 1.8 mm, and a light receiving plane of the first photodetector 277 receiving the ordinary light .alpha. is set at such a position at which the best focused image is formed.
It is practically impossible to receive the above spots by the photodetectors 277 and 278 each having four light receiving regions, because in general the light receiving regions are separated from each other by light insensitive regions having a width of about 10 .mu.m. Therefore, it is almost impossible to derive the focusing error signal. If the focusing error signal is obtained, an inclination of the focus error signal having a substantially S-shape becomes extremely steep near an in-focused position, i.e. a zero cross point, so that the focusing control could not be performed.
In the Publication of Kokai Hei 5-334760, there is explained that by setting the apex angle .gamma. of the second prism 272 smaller than 90.degree., a separation angle between the ordinary light .alpha. and the extraordinary angle .beta. can be enhanced and the astigmatism is increased. When the apex angle is set to be smaller than 90.degree., the exit surface of the second prism 272 is inclined with respect to the direction y, so that the astigmatism is increased and the separation angle between the ordinary light and the extraordinary light is also increased. Therefore, a size of the spot is increased and there is a possibility that the focusing error signal can be derived. However, a machining of the second prism 272 becomes very difficult and the ordinary light and extraordinary light might not be separated from each other due to an increase in the astigmatism.
In the Publication of Kokai Hei 5-334760, there is further stated that by using a concave lens in addition to the setting of the apex angle .gamma. smaller than 90.degree., it is possible to enlarge the separation angle between the ordinary light .alpha. and the extraordinary light .beta.. However, in this case a whole size of the optical head is liable to be increased due to the concave lens although the focusing error signal could be obtained. Moreover, by inclining the exit surface of the second prism 272, not only the astigmatism, but also the coma are introduced, so that it is rather difficult to derive the focusing error signal precisely.
In Japanese Patent Application Laid-open Publication Kokai Hei 5-22974, there is proposed another known optical head. In this known optical head, a laser beam emitted by a semiconductor laser 280 is made incident upon a half beam splitter 282 by means of a lens 281 and a beam reflected by a reflection surface of the half beam splitter is projected onto a magneto-optical record disk 284 by means of an objective lens 283 as a fine spot. A return laser beam reflected by the magneto-optical record disk 284 is made incident upon the half beam splitter 282 by means of the objective lens 283. A return beam transmitted through the half beam splitter 283 is made incident upon an optical element 286 by means of a lens 285.
The optical element 286 comprises first and second prisms made of optical materials having different refractive indices n1 and n2, and a polarizing film 297 provided at a boundary surface B. It should be noted that an entrance surface A of the optical element 286 is not parallel with an exit surface C. The optical element 286 is arranged such that the entrance surface A is perpendicular to an optical axis of an incident light beam and is inclined by 45.degree. with respect to the polarizing plane of the laser beam. Therefore, the return laser beam reflected by the magneto-optical record disk 284 is separated into a first beam transmitted through the polarizing film 287 and a second beam reflected by the polarizing film. Then, the first and second beams are made incident upon first and second photodetectors 288 and 289, respectively.
An information signal is derived from a difference between output signals from the first and second photodetectors 288 and 289 by the differential method. A focusing error signal is obtained from the output signals of first photodetector 288 by the astigmatic method. In order to obtain the focusing error signal correctly, a shape of the spots of the first and second beams on the first and second photodetectors 288 and 289, respectively has to be made ideal. To this end, the entrance surface A and the exit surface C of the optical element 286 are inclined each other so that the astigmatism is introduced only for the return beam and an introduction of the coma is suppressed.
In the known optical head illustrated in FIG. 4, the return beam reflected by the magneto-optical record disk 284 is separated into the first beam transmitted through the polarizing film 287 and the second beam reflected by the polarizing film 287, and these first and second beams are received by the first and second photodetectors 288 and 289, respectively. Therefore, the first and second photodetectors 288 and 289 are arranged to be separated from each other, and thus the optical head is liable to be large in size. Moreover, in order to obtain an ideal beam spot on the first photodetector 288, it is necessary to increase a distance P from a point at which a principal light ray of the incident light beam to the optical element 286 is made incident upon the polarizing film 287 to a point at which an imaginary focus point at which the light beam emanating from the exit surface C of the optical element 286 is converged. Said distance P should be nearly equal to 34 mm. Therefore, a size of the optical head is increased.
It may be considered that the coma may be corrected by utilizing the construction of the optical element 286 in the optical head shown in FIG. 2. However, such an application is practically very difficult due to the following reason. In the optical head illustrated in FIG. 4, it is essential that the refractive indices n1 and n2 of the first and second prisms of the optical elements differ from each other as can be understood from an equation described in the above mentioned Publication Kokai Hei 5-22974. The second prism 272 of the optical head shown in FIG. 2 is made of an anisotropic material having different refractive indices for the ordinary light and extraordinary light. Further, the refractive index for the extraordinary light varies in accordance with its propagating direction. Therefore, in the optical head shown in FIG. 2, the above mentioned equation described in the Publication Kokai Hei 5-22974 could not be satisfied.